SDH (synchronous digital hierarchy) and the related SONET are well known digital transport technologies, established in virtually every country in the world. When SDH was first conceived in the early 1990's, telecommunications traffic was predominantly voice. During the last years there has been an explosion in the demand for bandwidths driven mainly by internet excess, e-commerce and mobile telephony. This increase in demand has been satisfied through a combination of increased line rates (TDM—Time Division Multiplexing) and transmitting multiple wavelengths through a single fiber (DWDM—Dense Wave Division Multiplexing).
But as the network evolved to higher line rates, the physical limits of the transport medium (optical fiber) becomes critical. And, there remains an over-riding requirement to control the cost of providing an improving service to customers.
The latest recommendation from the ITU is G.709 “Interface for the Optical Transport Network” (OTN) which builds on the experience and benefits gained from SDH and SONET to provide a route to the next generation optical network. The OTN is therefore regarded as the lifeline to increased bandwidth capacity. Many of the concepts in ITU-T G.709 have their roots in SDH/SONET, for example a layered structure, in-service performance monitoring, protection and other management functions. However, some key elements have been added to continue the cycle of improved performance and reduced cost. These include, for example, management of optical channels in the optical domain.
ITU-T G.709 also provides a standardized method for managing optical wavelength (channels) end to end without the need to convert an optical signal into the electrical domain.
Because of the worldwide use of SDH/SONET technology, there will be a long period of coexistence of the SDH/SONET and the new OTN technology. Hence, the interworking of all these transmission technologies is mandatory for every company offering network elements for transmission networks. The most important network elements are, for example, DWDM (Dense Wavelength Division Multiplexer) systems, ADM (Add Drop Multiplexers) and cross-connects. Cross-connects are, for example, used for providing a connection between an SDH network and an OTH (Optical Transport Hierarchy) network.
This capability of handling transmission signals of two different layers (a layer represents a transmission technology like SDH or OTH) results in a network element having two subsystems, each assigned to one layer. The exchange of signals between both subsystems is achieved by dedicated interconnections between the subsystems.
It is apparent that the aspect of managing such a network element becomes more and more important with the growing complexity and functional integration of such network elements. This has resulted in industry-wide pressure to adopt standardized management interfaces on telecommunications equipment. SDH was, for example, the first major new technology where management features have been incorporated in supporting standards. A detailed overview of management features can be found in the paper “Management of SDH Network Elements: An Application of Information Modelling”, O. De Romemont et al., in Electrical Communication—4th Quarter 1993, pages 329-338. The contents of this paper is herewith incorporated herein by reference.
In this paper, an OSI system management framework is described providing an overall management model, a generic information model, a methodology for definition of management information and a management protocol for the purpose of communicating management information between two open systems. According to the OSI system management model, a system is composed of a set of resources that exist to provide services to a user. These resources may exist independently of their need to be managed. System management defines a management view of a resource as a managed object (MO), which represents the resource, for the purpose of management, at the interface of the system. The managed object acts as the recipient for the management operations issued by the manager and is responsible for sending reports related to spontaneous events that happen in the system.
All the relevant data is thus encapsulated within MOs and can only be referenced or changed by the defined methods of the MOs. MOs are somewhat specialized compared to the objects in a typical object-oriented approach because they reflect the asymmetry of the manager/agent relationship.
The complete set of managed objects in a managed system constitutes the management information model (management information base) and completely represents the management information that the agent exhibits at its interface to an operation system. These principles have been widely applied in the network element software used by a network element control system. Managed objects within a network element, like the above-mentioned cross-connect, are, for example, the subsystem handling SDH transmission signals and the subsystem handling OTH transmission signals.
Further to the control system of the network element, there is a higher level network management system which is responsible for the management of one or more network elements. Hence, the network element viewpoint is concerned with information that is required to manage a network element as viewed on an individual basis. It provides the capability to install, commission and bring into service the physical and logical resources of the network element and makes them available to higher level management application. This viewpoint is restricted to local information contained within one network element and contains no information relating to connectivity outside the network element. In contrast thereto, the network management viewpoint is concerned with information representing the network both, physically and logically. It is concerned with how network entities are related, topographically interconnected and configured to provide and maintain and to end transport network services.
Referring now again to the above-mentioned cross-connect, its network element control system serves to handle the SDH and OTH subsystems. If, however, a signal has to be sent from one subsystem to the other subsystem, then the network management system of the network element has to select one interconnection between both subsystems and has to configure the subsystems accordingly. This requires that the network management system has knowledge about the number of interconnections between both subsystems within the network element and the status of each of them (already used or not yet used). Hence, this exposes implementation details of the network element to the network management system and makes management of a network more complex.
In prior art approaches, the management system obtains the knowledge about the interconnections between the OTH and SDH subsystems either via explicit configuration by the operator or by requesting this knowledge from the network element control system. Hence, this approach leaves the decision which interconnection to use and the related complexity with the management system.